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a Chapter 4 Spectral Doppler Sonography: Waveform Analysis and Hemodynamic Interpretation 43
Fig. 4.10. Flow quantification
by time domain processing.
Top: Velocity waves. Bottom:
M-mode imaging of the flow
and the outlining of effective
flow lumen. Maximum velocity,
minimum velocity, and average
flow are measured for one cardiac
cycle as defined by the
cursors 1 and 2
movement are being investigated as a non-Doppler
sonographic technique for measuring velocity and
therefore blood flow. Speckle patterns remain mostly
stable between B-mode image frames and tracking
their frame-to-frame displacement allows velocity
measurement in a flow (Fig. 4.11) [18]. Theoretically,
the technique is capable of measuring both the axial
and the lateral flow velocity vectors unencumbered by
the angle of insonation, aliasing, or range ambiguities.
However, the accuracy of speckle tracking is less
for the lateral vectors than for the axial vector aligned
with the beam path and several techniques exist for
improving the lateral tracking [19]. The speckle
tracking principles may also be applied for measuring
volumetric flow in the emerging three- or four-dimensional
sonography [20]. This approach holds exciting
possibilities for real-time flow and perfusion
assessment in clinical settings. Speckle tracking technology
is not yet commercially available.
Doppler Waveform Analysis
and Doppler Indices
Because of the problems related to volumetric flow
assessment, there has been a need to seek alternative
ways to investigate vascular flow dynamics using the
Doppler method. The maximum Doppler frequency
shift waveform represents the temporal changes in
the peak velocity of RBC movement during the cardiac
cycle. It is therefore under the influence of both
Fig. 4.11. Graphic depiction of speckle tracking velocimetry.
The best match of a speckle target region in successive
frames determines the distance traveled in unit time. This
forms the basis of measuring the velocity and therefore
the flow
upstream and downstream circulatory factors [21].
The objective has been to obtain information specifically
on distal circulatory hemodynamics. Techniques
have been developed for analyzing this waveform in
an angle-independent manner. Most of these analytic
techniques involve deriving Doppler indices or ratios
from the various combinations of the peak systolic,
end-diastolic, and temporal mean values of the maximum
frequency shift envelope (Fig. 4.12). Because
these parameters are obtained from the same cardiac